1. Field of the Invention
The invention relates to a magnetooptic recording medium which is used in a magnetooptic disk device or the like and from which information that has been recorded by magneto-optical effect such as magnetic Kerr effect, Faraday effect, or the like can be read out.
2. Related Background Art
Hitherto, in the magnetooptic recording medium, it is difficult to find out a single magnetic material in which a Curie temperature is low, the recording can be easily performed, a coercive force is high, a preservation stability is high, further, a magneto-optical Kerr rotational angle is large, and good reading characteristics are obtained. Therefore, a magnetooptic recording medium in which the necessary functions are separated and two different magnetic materials are laminated has been proposed in U.S. Pat. No. 4,799,114.
FIG. 1 is a schematic cross sectional view of a conventional magnetooptic recording medium using an exchange coupling film. In the diagram, reference numeral 11 denotes an optical transparent substrate made of a glass or plastics. An under layer 12 made of an inorganic dielectric material such as SiN.sub.X or the like to obtain an interference effect and a corrosion preventing effect is provided on the substrate 11. Further, a magnetic layer 13 serving as a reproducing layer and a recording layer 14 having a larger coercive force and a lower Curie temperature than those of the magnetic layer 13 are formed on and over the under layer 12. A dielectric layer 15 made of SiN.sub.X or the like for preventing the corrosion of the magnetic layer and for producing an interference effect is formed on the recording layer. Those films are continuously formed without breaking a vacuum state.
Further, in U.S. Pat. No. 4,753,853, there has been proposed a magnetooptic recording medium which uses a rare earth--iron group amorphous alloy whose iron group sublattice magnetization is dominant as a layer of a high coercive force and which also uses a rare earth--iron group amorphous alloy whose iron group sublattice magnetization is dominant as a layer of a low coercive force and in which the saturation magnetizing directions of both of the alloys are set to be parallel. As a medium having an excellent information stability, a medium using a rare earth--iron group amorphous alloy having a compensation temperature between the room temperature and the Curie temperature as a layer of a high coercive force has been proposed.
In recent years, a recording method which is used in a magnetooptic disk having a large capacity is a method whereby recording mark lengths in the inner periphery and outer periphery of the disk in case of the CAV (Constant Angular Velocity) disk are different. Although a further large capacity is demanded in future, an M-CAV (Modified CAV) method is known as a recording method which can accomplish such a demand. In the M-CAV method, information is recorded by a predetermined mark length, that is, linear velocities or recording frequencies in the inner and outer peripheral portions of the disk are made different. A pit edge recording method whereby information of "0" and "1" are made correspond to the edge portions of the recorded pits has also been proposed.
Although the overwriting function is, further required to improve a data transfer speed, a magnetic field modulation overwriting method is considered to be hopeful as a method which can satisfy such a requirement. In the magazine of The Japan Society of Applied Magnetism, 2PB-7, 1988, there is a report such that in the magnetic field modulation overwriting method, a medium structure having a metal layer such as aluminum or the like having a high heat conductivity has better characteristics such as a C/N ratio and the like.
In the magnetooptic recording medium using two films of the exchange coupling layers which has hitherto been being examined, since a film thickness of magnetic layer is so thick to be a value within a range from 800 to 1000 .ANG., there are problems such that a heat capacity is large and, in the M-CAV recording mode, in a region of a high linear velocity, a laser power which is necessary in the recording mode is large, so that it is not practical.
To reduce the laser power necessary in the recording mode, it is sufficient to make the film thickness of magnetic layer thin. When the magnetic layer is made thin, however, a new problem such that a magnetooptic signal decreases occurs. In U.S. Pat. No. 4,628,485, therefore, a magnetooptic recording medium in which two exchange coupling layer films are used as magnetic layers and, further, a reflecting film is provided to increase a magneto-optical effect, namely, a Kerr rotational angle has been proposed.